This invention relates generally to methods for growing layers on wafers and more particularly to growing layers of different materials in different growth chambers.
As is known in the art, wafers such as semiconductor wafers can have advanced layer structures that incorporate several different materials. Typically, these structures are accomplished by depositing the layers on the wafer in one growth apparatus or chamber such as a molecular beam epitaxy (MBE) machine. There are, however, situations where forming these layers in one chamber is undesirable. In one situation, background cross contamination of layers is possible. For example, if a III-V layer and a II-VI layer are grown in one chamber, then the high vapor pressure of group V elements (i.e., elements in group V of the Periodic Table, e.g., Arsenic As, and Phosphorus P) can unintentionally dope the II-VI film and contaminate the II-VI furnaces. Arsenic in a first-deposited layer can evaporate and undesirably combine with elements for a second-deposited layer. The high vapor pressure group VI elements (e.g., Sulfur S, Selenium Se, and Tellurium Te) can also contaminate the III-V film and can contaminate III-V furnaces. Another undesirable situation is when different capabilities are required for growing different layers. For example, equipment limitations may prevent one chamber from growing different layers that require, e.g., different temperatures. Also, undesirable amounts of time and/or money may be necessary to establish different capabilities for growing two different layers in one growth chamber.
The problems associated with growing different material layers in one growth chamber mean that it is often desirable, if not required, to use different growth chambers to grow layers of different materials on a wafer. In this case, one layer is grown in one chamber and the wafer is transferred to a second chamber to grow the second layer on the wafer. This typically requires exposing the wafer to the atmosphere during transfer between chambers, although in rare cases chambers are connected by vacuum tubes. Exposure to the atmosphere causes significant surface contamination that cannot be eliminated by subsequent wafer outgassing in the second growth chamber.
To prevent contamination of the wafer surface when transferring the wafer between chambers, the wafer surface can be coated with a protective layer. In this case, a first layer is deposited on the wafer in a first chamber. Then, for example, an arsenic protective layer is deposited over the first layer in the first chamber. The wafer is removed from the first chamber and transported to a second chamber where the arsenic layer is removed (i.e., desorbed) by heating the wafer. With the arsenic protective layer substantially removed, a second layer is deposited over the first layer on the wafer. Using this technique has the advantage that atmospheric contamination is prevented when transferring the wafer from the first chamber to the second chamber.
Using an arsenic protective layer when transferring the wafer between chambers, however, has several disadvantages. Arsenic does not begin to adsorb onto some surfaces, e.g., semiconductor surfaces, until the wafer temperature is below 100xc2x0 C. Therefore the wafer temperature must be cooled to below 100xc2x0 C. from temperatures of 560xc2x0 C. and higher used when depositing the layer to be protected, requiring cooling times of an hour or more. During this lengthy cooldown period, the wafer surface can be contaminated from materials present in the chamber. For reactive surfaces such as aluminum gallium arsenide, background contamination during cooldown is also a significant problem. Additionally, if the layer to be protected by the arsenic protective layer includes arsenic, then arsenic is supplied to the surface of the wafer during cooldown to replace arsenic that evaporates from the wafer surface. Supplying arsenic during the lengthy cooldown thus increases the arsenic consumption. Still more time, e.g., 20 minutes, is required to deposit the arsenic protective layer on the wafer. Also, arsenic often does not bond strongly to the wafer surface, which allows the arsenic to flake off if the wafer is vibrated or bent during handling when being transferred between chambers.
In general, in one aspect, the invention provides a method of fabricating a wafer including growing a single crystal layer comprising a III-V compound in a first chamber at a temperature above 350xc2x0 C. A temperature of a surface of the single crystal layer is reduced to below about 350xc2x0 C. in the first chamber. An indium arsenide layer is deposited on the single crystal layer, to form an intermediate structure, in the first chamber at a temperature below 350xc2x0 C. and above 100xc2x0 C. The intermediate structure is transferred to a second chamber. A surface of the intermediate structure is heated to a temperature above about 600xc2x0 C. to remove substantially all of the indium arsenide layer and impurities collected in the indium arsenide layer during the transfer to the second chamber. Another material is deposited on the single crystal layer in the second chamber.
With such an arrangement, impurities collected by the wafer during transfer between chambers can be removed, while requiring less time to deposit a protective layer on the wafer than if an arsenic-only protective layer is deposited on the wafer.
In general, in another aspect, the invention provides a method including depositing a layer of a first material on a single crystal substrate in a first chamber. A second material is deposited in the first chamber over a surface of the layer at a temperature above 100xc2x0 C., the second material including one or more elements each of which is in a Periodic Table group of an element in the first material. The second material is heated in a second chamber to substantially remove the second material.
Embodiments of the invention may have the first material include a III-V compound and the second material include indium arsenide.
In general, in another aspect, the invention provides a method of processing a wafer and protecting the wafer from impurities. The method includes growing a III-V layer in a first chamber on a substrate. An indium arsenide layer is deposited on the III-V layer in the first chamber. The indium arsenide layer is heated in a second chamber to desorb the indium arsenide layer. Another layer is grown on the III-V layer in the second chamber.
Embodiments of the invention may have the second temperature lower than the first temperature and above 100xc2x0 C.
In general, in another aspect, the invention provides a method of fabricating a wafer, the method including growing a single crystal layer including a III-V compound in a first chamber at a first temperature over a substrate. An indium arsenide layer is deposited over the single crystal layer, to form an intermediate structure, in the first chamber at a second temperature lower than the first temperature and above 100xc2x0 C. The intermediate structure is transferred to a second chamber where the indium arsenide layer is desorbed at a third temperature greater than the second temperature.
Various embodiments of the invention may provide one or more of the following advantages. For gallium arsenide growth at about 560xc2x0 C., less than 10 minutes of cooling time can be used prior to applying a protective coating of e.g., indium arsenide. In situ gettering of contamination can be significantly reduced compared to conventional means. A 50 xc3x85 layer of indium arsenide can be applied as a protective coating in less than one minute. Throughput of wafers can be maintained while providing an indium arsenide protective coating for transit between chambers. Reduced amounts of arsenic can be consumed during cooling prior to applying a protective coating compared to applying arsenic protective layers. A protective coating of, e.g., indium arsenide can be applied to gallium arsenide layers, for protection during transit between chambers, that bonds better to the gallium arsenide than an arsenic protective coating.